The results of all this is Nexus. This is a bit of a "kitchen sink" setting where just about anything can exist somewhere in the world. This is sort of like the Infinity Unlimited setting, but with "lost technology". There are lots of races, monsters, magic systems, guns, and even some sci-fi elements.
The premise is that long ago, a race of now lost being created a stable network of portals to other worlds. From these worlds other races came and established their own regions. At some point in the past the portal creators disappeared, and later the portals stopped working, leaving the many races unable to return to their home worlds. Rumors of working portals still exist, and many adventurers make a career of just seeking out these portals or other lost magics/technologies.
While most of the world is going to appear as a late middle ages, there could be just about anything out in the "unknown lands". There are even areas that are in "flux" where the portals didn't quit shut down and instead created a strange area that transforms back and forth between the "local" and "other" worlds.
So, as you can see, just about anything can be thrown into this world. This gives me an excuse to freely pilfer from myth, fantasy, sci-fi, and still have everything feel like it is consistent with the world. The starting "known world" is going to be a fairly stable middle ages type area, but even here there can be pockets of non-standard fantasy or sudden new transformative elements.